Large extensional aftershocks in the continental forearc triggered by the 2010 Maule earthquake, Chile

The M w 8.8 Maule earthquake occurred off the coast of central Chile on 2010 February 27 and was the sixth largest earthquake to be recorded instrumentally.This subduction zone event was followed by thousands of aftershocks both near the plate interface and in the overriding continental crust.Here, we report on a pair of large shallow crustal earthquakes that occurred on 2010 March 11 within 15 min of each other near the town of Pichilemu, on the coast of the O'Higgins Region of Chile.Field and aerial reconnaissance following the events revealed no distinct surface rupture.We infer from geodetic data spanning both events that the ruptures occurred on synthetic SW-dipping normal faults.The first, larger rupture was followed by buried slip on a steeper fault in the hangingwall.The fault locations and geometry of the two events are additionally constrained by locations of aftershock seismicity based on the International Maule Aftershock Data Set.The maximum slip on the main fault is about 3 m and, consistent with field results, the onshore slip is close to zero near the surface.Satellite radar data also reveal that significant aseismic afterslip occurred following the two earthquakes.Coulomb stress modelling indicates that the faults were positively stressed by up to 40 bars as a result of slip on the subduction interface in the preceding megathrust event; in other words, the Pichilemu earthquakes should be considered aftershocks of the Maule earthquake.The occurrence of these extensional events suggests that regional interseismic compressive stresses are small.Several recent large shallow crustal earthquakes in the overriding plate following the 2011 M w 9.0 Tohoku-Oki earthquake in Japan may be an analogue for the triggering process at Pichilemu.